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Rock is denser then gas so before it got anywhere near that mass it would collapse on itself and be turnend into a black hole.

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There's a critical mass at which point a planet would become a star first.

That said, planets don't "become stars".

At the formation of our solar system, as the gasses coalesce into pre-celestial bodies, the amount of mass determines the bodies' courses of actions. Below a certain point and you end up a planet. Above a certain point and you become a star.

If there were enough gas to form a giant star, then not only is it highly unlikely that there would be planets, but it is a near impossibility that there would be life. This is the result of the fact that large stars burn their fuel fast. Stars that are large enough to become black holes are VERY large and thus burn their fuel VERY fast (like, in millions of years and not billions). This time frame is just not long enough to allow life to develop on the planets in the solar system.

Hmm..well, the thing is, I'm not sure we've figured out which of the chemical processes can support an evolutionary process... and whatever chemical it is, I suppose it's got to be in liquid phase in temperature...or in solution...or some thing...and then there's pressure to worry about, gravity, solidity of structure, gaseous intake or output, energy consumption, locomotion...growth, decay, yadda, yadda, yadda...

Oops. My bad. Venus does, indeed, weigh less. That's sort of what I wanted to say. Poor proof reading on my part. When I use the iPad I can't type as fast as I like and editing posts is a pain in the ass.

So...what would happen to a rocky planet that got to big? Would it just stay molten? Or would it tear itself apart because of gravity? In an extreme case...could you get fusion?

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Tear itself apart? No. The bigger it gets, the more gravity. So it would only stay together. But, if enough gas, meteors and comets continued to fall onto it then eventually the mass would become so great that the matter at it's core would begin to create a nuclear reaction. But with so many heavy elements it would be little more than a brown dwarf.

Tear itself apart? No. The bigger it gets, the more gravity. So it would only stay together. But, if enough gas, meteors and comets continued to fall onto it then eventually the mass would become so great that the matter at it's core would begin to create a nuclear reaction. But with so many heavy elements it would be little more than a brown dwarf.

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I'm no astrophysicist, but I do know that lighter elements give rise to heavier ones, which means that the vast majority of the matter in the universe is composed of hydrogen and helium. (about 75% of the mass is hydrogen alone). So I'd imagine that the chance of a solid planet ever reaching a mass anywhere near that required to initiate fusion would be vanishingly small.

I'm no astrophysicist, but I do know that lighter elements give rise to heavier ones, which means that the vast majority of the matter in the universe is composed of hydrogen and helium. (about 75% of the mass is hydrogen alone). So I'd imagine that the chance of a solid planet ever reaching a mass anywhere near that required to initiate fusion would be vanishingly small.

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Incorrect. The universe, at one time, was all lighter elements (Hydrogen). When the first stellar masses began coalescing--about 13 billion years ago--they formed huge stars, many millions of time larger than our star. They lived but a few hundred thousand years before collapsing in on themselves, fusing the larger elements and then exploding them across the cosmos.

This process still takes place to this day, but on a smaller scale. Betelgeuse, which is pretty close as super-giants go, won't last but a few million years more. In its death, the supernova will fuse elements heavier than Carbon and Oxygen. It'll make very small amounts of the heavier elements, along with a good number of the middle elements, which it will scatter when it dies.

As for your second point, I agree, the possibility of a planet becoming a star is vanishingly small, though possible AFAIK.

go ahead. how about the size of vy canis majoris. our sun is invisible next to it, that's how large it is. how about a rocky planet that size or larger?

perhaps it would have a larger sun or several smaller suns to support life.

if a planet existed that size, it may be almost impossible to travel around the planet in one's lifetime.

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"VY Canis Majoris (VY CMa) is a red hypergiant star located in the constellation Canis Major. At between 1800 and 2100 solar radii (8.4–9.8 astronomical units, 2.7 billion km or 1.7 billion miles in radius), it is currently the largest known star and also one of the most luminous known."http://en.wikipedia.org/wiki/VY_Canis_Majoris

I said a super Earth 1,000 times the size of Earth. You are proposing I plausibly explain a super Earth tens of millions of times the size of Earth- I never said that, did I? But you know what's hundreds of millions of times the size of Earth?

Solid-skin Dyson spheres. They're not impossible to build either, just very very hard to engineer- this is 30 or 40th century science.

Solid-skin Dyson spheres. They're not impossible to build either, just very very hard to engineer- this is 30 or 40th century science.

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Solid Dyson spheres are not possible, because their position relative to the star at the centre is unstable. What is possible is a ruck load of satellites that orbit the sun independently, which is known as a Dyson swarm.